1. Field of the Invention
The present invention relates to an image-fixing method, and in particular to an image-fixing method suitable for use in machines making use of electrophotographic processes such as copying machines, printers, and facsimiles.
2. Description of the Related Art
Permanent images are formed in an electrophotographic process, during copying, by forming an electrostatic latent image on a photoreceptor made of a photoconductive material, developing the latent image with a toner into a toner image using, for example, a magnetic brush method, transferring the toner image on the photoreceptor onto a recording medium (transfer medium) such as paper or a sheet, and then fixing the image using heat, solvent, or pressure.
Heat melting processes have been most widely used for fixing transferred toner images and are classified largely into two processes: contact processes and non-contact processes. In particular, contact heat-roll fixing processes, which are superior in thermal efficiency and thus allow high-speed fixing, have been widely employed recently in commercial copying machines, printers, etc. However, heat-roll fixing processes have some drawbacks. It has been pointed out that one of the most serious drawbacks is that they require a longer standby time (warming-up time) before use.
Heat-fixing processes are of course far superior to pressure-fixing processes, which are ready to operate immediately after power has been turned on, in durability of fixed images formed on a recording medium such as paper and in resistance to deformation, cockle generation, etc. of the paper due to pressure. Accordingly, methods for shortening the standby time of heat-roll fixing processes have been intensively studied.
One of the promising approaches for achieving the object above is to use a toner binder resin that, for example, has a glass transition temperature (Tg) of several tens of degrees lower than, or has a molecular weight lower than, that of commonly used resins. However, many such toners have a fatal drawback in that they often aggregate or exhibit a caking phenomenon during storage or in a copying machine.
To solve the problem above, a method was proposed of adhering very fine particles such as colloidal silica, alumina, or titania to the surface of toner particles for improvement in blocking resistance and flowability. The method does not demand a significant increase of the lowest fixing temperature and improves the blocking resistance and flowability of the toner to some extent. However, the fine particles, even if subjected to heat treatment or the like for adhesion to the toner particle surface, are often released from the toner particle surface, negatively affecting the photoreceptor, in particular one having a surface coated with an organic polymer or the like. In other words, the method leads to the disadvantage that the fine particles are adhered semipermanently to the photoreceptor surface during repeated use, causing the trouble of image defects. Therefore, the method is not an ultimate solution.
In addition, use of the binder resin described above in the heat-roll fixing process leads to the problem of occurrence of an offset phenomenon, wherein toner particles are adhered to a heating roll and the adhered toner particles stain the next copy, because of the resin's inherent thermal properties. In particular, increasing the amount of heat applied per unit time to cope with increased fixing speed during high-speed copying results in a higher incidence of the offset phenomenon.
Use of various waxes in large amount as the fixing aid for the toner has also been proposed. However, these methods also carry the drawbacks of the toner particle-caking phenomenon described above and other secondary problems.
In a heat-roll fixing process, curling of a transfer medium, generally paper, around a fixing roll after fixing is ordinarily prevented by a stripper finger placed in the fixing roll unit (heating unit). However, with the recent trend toward higher-speed copying machines, stress applied to this site has become larger, leading to more frequent trouble including inadequate release and image defects at the edge of the transfer medium caused by the finger tip during release.
Exfoliation or local detachment of the surface layer of fixing units due to inadequate release and excessive application of local stress further causes fatal defects in the surface of the fixing units (heating unit and pressurizing unit) surface. For example, the surface layer of the heating unit is usually coated with a layer of polymer superior in release properties such as a silicone or fluorocarbon resin for prevention of toner particle adhesion, and if the layer is damaged, toner components remain on the surface of the heating unit. This causes the offset phenomenon wherein toner is retransferred onto an unintended printing face.
On the other hand, induction-heating processes have been proposed as fixing methods for shortening the standby time before use (for example, Japanese Patent Application Laid-Open (JP-A) Nos. 58-178385, 59-33785, and 59-33788). Some of the processes are now being practically applied, but the processes still required coating of the metal roll heated by induction with, for example, a silicone or fluorocarbon resin for ensuring toner-releasing properties. Also, defects in the fixed coat layer, generated during repeated use by unintended paper jamming or the like, could not be prevented, resulting in a drastic decrease in reliability.
The present invention is made in view of these circumstances in the related art to overcome the problems. Thus, the invention is devised to provide an image-fixing method applicable to more resource- and energy-saving fixing systems that shortens standby time without an increase in power consumption and that eliminates degradation of a fixing unit surface even with long-term repeated use or after paper jamming.